A Clean Alternative or a New Environmental Challenge? A Perspective on Downstream Emissions and Environmental Trade-Offs of Ammonia-Diesel as a Low-Carbon Future Fuel.

The use of ammonia (NH3) as a carbon-free fuel in non-road engines, e.g. for maritime traffic or agricultural machines, emerges as an option to reduce carbon dioxide (CO2) emissions. However, the environmental consequences of large-scale NH3 combustion extend beyond CO2 mitigation and remain insufficiently explored. This perspective briefly discusses the benefits and drawbacks of different NH3 combustion technologies and highlights key challenges, including combustion byproduct formation, incomplete NH3 conversion, secondary aerosol formation, and potential toxicological impacts for NH3 with diesel pilot fuel injection. NH3 combustion may introduce reactive nitrogen species and organic byproducts that affect atmospheric chemistry, air quality, and public health. The interaction of NH3-derived emissions with biogenic and anthropogenic pollutants adds complexity to their environmental fate. Given these uncertainties, early stage assessments of emissions, atmospheric transformations, and their downstream effects are mandatory. By integrating combustion science, atmospheric chemistry, and toxicology, a proactive approach is suggested to identify and mitigate unintended consequences before NH3 becomes a dominant energy carrier.